AldiTalk also has an interesting roaming bundle for data in other EU countries which gives you 150MB valid for 7 days costing € 4.99. Once consumed you can reorder a new bundle minutes later. Otherwise data will start to be billed volume-based at € 0.24/MB.
Of course data would be much cheaper if you get a separate Austrian SIM card for Austria.

Generally, I agree with Inquisitor about ALDItalk as one of the best solutions.
If you can't find any ALDI market in your area, because in the centers they are not all over, then you can go to any other supermarket chain (Lidl, Rewe, Edeka, Penny, Plus), electronic market (Media Markt, Saturn) or gas station. But try to stay away from the shops of the network providers (Telekom, Vodafone, eplus/Base, o2) as they will charge you twice.

Unfortunately, you can't put the SIM in your phone and just have connection. I've just registered an ALDItalk SIM for a Russian friend. Not speaking any German, he was not able to do it alone.

This I find very annoying: you will need to register your SIM first, before you have connection:
a.) either with their customer support by phone (but hey! you don't have phone connection yet) and don't know about the English skills of the support hotline.
b.) or on their website, but this is only available in German and you are not likely to have internet already. You may log on a wifi in your hotel. And with ALDI you will be guided through the whole portfolio in German. For the 500 MB pack look for "Internet-Flatrate M" and "Option buchen".

For me it's kind of crazy to give new customers only two ways of registering, but both ways require exactly the way of communication you want to establish, but can't so far, because of not being registered.

I shouldn't mention that: of course you can give any German address (or even name) you can think of, because your data will not be verified. And yes: It takes some time, until your registration is "processed" and you will have network or data. With ALDI it took me about 30 minutes, some other networks can take hours.

To ways out of the mess:
If you require an English speaking website (and can't handle Google Translate) take Lebara mobile or Lycamobile. They have good rates too.
If you require personal attention registering, go to an small cell phone "Handy" shop where they sell these or similar SIM cards, not to supermarkets or gas stations where you can't expect any support at all (not even in German).

Hey,
I visited Munich during Oktoberfest last year. Me and my wife wanted to stay in touch with our daughter who gave birth to her first daughter We asked around and our German friends told us that O2 was the cheapest solution. However we then decided to use wifi at our friend's house instead of SIM-card so I do not know if O2 is really the best. Hope this helps!

Generally, I agree with Inquisitor about ALDItalk as one of the best solutions.
If you can't find any ALDI market in your area, because in the centers they are not all over, then you can go to any other supermarket chain (Lidl, Rewe, Edeka, Penny, Plus), electronic market (Media Markt, Saturn) or gas station. But try to stay away from the shops of the network providers (Telekom, Vodafone, eplus/Base, o2) as they will charge you twice.

Unfortunately, you can't put the SIM in your phone and just have connection. I've just registered an ALDItalk SIM for a Russian friend. Not speaking any German, he was not able to do it alone.

This I find very annoying: you will need to register your SIM first, before you have connection:
a.) either with their customer support by phone (but hey! you don't have phone connection yet) and don't know about the English skills of the support hotline.
b.) or on their website, but this is only available in German and you are not likely to have internet already. You may log on a wifi in your hotel. And with ALDI you will be guided through the whole portfolio in German. For the 500 MB pack look for "Internet-Flatrate M" and "Option buchen".

For me it's kind of crazy to give new customers only two ways of registering, but both ways require exactly the way of communication you want to establish, but can't so far, because of not being registered.

I shouldn't mention that: of course you can give any German address (or even name) you can think of, because your data will not be verified. And yes: It takes some time, until your registration is "processed" and you will have network or data. With ALDI it took me about 30 minutes, some other networks can take hours.

To ways out of the mess:
If you require an English speaking website (and can't handle Google Translate) take Lebara mobile or Lycamobile. They have good rates too.
If you require personal attention registering, go to an small cell phone "Handy" shop where they sell these or similar SIM cards, not to supermarkets or gas stations where you can't expect any support at all (not even in German).

I was looking at that Wikia page and it only outlines the offers for the MVNOs, not the actual network operators. It says the MVNOs have a better deal but when you say the network operator shops will "charge you twice" are you saying their prepaid offers are double the price of the MVNOs?